Re: [PATCH 00/33] hw/cpu/arm: Remove one use of qemu_get_cpu() in A7/A15 MPCore priv

[Markus Armbruster <armbru@redhat.com>]

Peter Xu <peterx@redhat.com> writes:

> On Wed, Jan 10, 2024 at 07:03:06AM +0100, Markus Armbruster wrote:
>> Peter Xu <peterx@redhat.com> writes:
>> 
>> > On Tue, Jan 09, 2024 at 10:22:31PM +0100, Philippe Mathieu-Daudé wrote:
>> >> Hi Fabiano,
>> >> 
>> >> On 9/1/24 21:21, Fabiano Rosas wrote:

[...]

>> >> > No one wants
>> >> > to be policing QOM hierarchy changes in every single series that shows
>> >> > up on the list.
>> >> > 
>> >> > Anyway, thanks for the pointers. I'll study that code a bit more, maybe
>> >> > I can come up with some way to handle these cases.
>> >> > 
>> >> > Hopefully between the analyze-migration test and the compat tests we'll
>> >> > catch the next bug of this kind before it gets merged.
>> >
>> > Things like that might be able to be detected via vmstate-static-checker.py.
>> > But I'm not 100% sure, also its coverage is limited.
>> >
>> > For example, I don't think it can detect changes to objects that will only
>> > be created dynamically, e.g., I think sometimes we create objects after
>> > some guest behaviors (consider guest enables the device, then QEMU
>> > emulation creates some objects on demand of device setup?),
>> 
>> Feels nuts to me.
>> 
>> In real hardware, software enabling a device that is disabled by default
>> doesn't create the device.  The device is always there, it just happens
>> to be inactive unless enabled.  We should model the device just like
>> that.
>
> It doesn't need to be the device itself to be dynamically created, but some
> other sub-objects that do not require to exist until the device is enabled,
> or some specific function of that device is enabled.  It is logically doable.
>
> Is the example Cedric provided looks like some case like this?  I am not
> sure, that's also why I'm not sure the static checker would work here.  But
> logically it seems possible, e.g. with migration VMSD needed() facilities.
> Consider a device has a sub-function that requires a sub-object.  It may
> not need to migrate that object if that sub-feature is not even enabled.
> If that object is very large, it might be wise to do so if possible to not
> send chunks of junk during the VM downtime.
>
> But then after a 2nd thought I do agree it's probably not sensible, because
> even if the src may know whether the sub-object will be needed, there's
> probably no good way for the dest QEMU to know.  It can only know in
> something like a post_load() hook, but logically that can happen only after
> a full load of that device state, so might already be too late.
>
> Thanks,

If an object has state that needs to be migrated only sometimes, and
that part of the state is large enough to bother, we can put it in an
optional subsection, can't we?

Destination: if present, take it.  If absent, initialize to default.

Source: send unless (known to be) in default state.